The present invention relates to a heat transfer plate, according to the preamble to claim 1, and a plate pack for use in a plate heat exchanger. The invention further relates to a plate heat exchanger consisting of such plates and plate packs respectively.
A plate heat exchanger comprises a plate pack consisting of a number of assembled heat transfer plates forming plate interspaces between them. Usually, every second plate interspace is connected with a first inlet duct and a first outlet duct, each plate interspace being arranged to define a flow area and to convey a flow of a first fluid between said inlet and outlet ducts. Correspondingly, the other plate interspaces are connected with a second inlet duct and a second outlet duct for a flow of a second fluid. Thus, the plates are in contact with one fluid through one of their side surfaces and with the other fluid through the other side surface, which allows a major heat exchange between the two fluids.
Modern plate heat exchangers have heat transfer plates, which in most cases are made of thin sheet that have been pressed and punched to obtain their final shape. Each heat transfer plate is usually provided with four or more xe2x80x9cportsxe2x80x9d formed by through holes being punched in the plate. The ports of the different plates define said inlet and outlet ducts, which extend through the plate heat exchanger transversely of the plane of the plates. Gaskets or any other form of sealing means are arranged around some of the ports alternately in every second plate interspace, and in the other plate interspaces around the other ports to form the two separate ducts for the first and the second fluids respectively.
Since the fluid pressure levels attained in the heat exchanger during operation are considerable, the plates need to have a certain stiffness so as not to be deformed by the fluid pressure. The use of plates made of sheet bars is possible only if these are somehow supported. As a rule, this is solved by the heat transfer plates being designed with some kind of corrugation so that the plates abut against each other in a large number of points. The plates are fixed to each other between two stiff end plates in a xe2x80x9crackxe2x80x9d and thereby form stiff units with flow ducts in each plate interspace. To obtain the desired contact between the plates two different types of plates are manufactured, which are sandwiched in such manner that the plates in the heat exchanger are alternately of a first kind and of a second kind.
A modern example of a plate heat exchanger of this type is disclosed in U.S. Pat. No. 5,226,474. This plate heat exchanger is intended for evaporation of a liquid fluid taken in through a central inlet at the lower edge of each plate and discharged from the plate heat exchanger, in the form of vapour and concentrated liquid fluid, through an outlet located at one upper corner of the plate. The second fluid is taken in in the form of vapour through an inlet located at the other upper corner of the plate and discharged in the form of condensate and residual vapour through two outlets located at the two lower corners of each plate.
The manufacture of a plate heat exchanger of this type requires two different types of plates, which means that two sets of pressing tools are needed, which in turn implies big investments. The need for two different types of plates also means that large storage space is needed, both for the finished plates and for the pressing tools. Furthermore, the tools have to be changed in connection with the pressing of the plates.
Ever since the manufacture of plate heat exchangers with heat transfer plates made of sheet bars started, a solution which means that only one type of plate is needed has been in demand in the industry, since this would be more cost-effective.
There are some cases today where only one type of plate is used, for example in applications subject to two fundamental design requirements: on the one hand that inlets and outlets for each heat exchanging fluid can be located at the same lateral edge of the plates and, on the other, that the plates can be designed in such manner that size of the inlets and outlets is the same for the respective fluids. An example of such a plate is disclosed in U.S. Pat. No. 4,359,087.
In such specific cases, it is ensured that the ports and sealing elements of the plates, such as weld-prepared ridges and/or gaskets, are arranged symmetrically about a symmetry line located in the centre of the plate between the inlet and the outlet of the two fluids and extending transversely of the main flow direction of the fluids. The plates of the heat exchanger are arranged in such a way that every second plate is rotated or xe2x80x9cflippedxe2x80x9d through 180 degrees about its symmetry line. The location requirement concerning the inlets and outlets is due to the fact that the location of the sealing means relative to the ports that define the inlet and outlet ducts has to be the same for all plates. Locating the inlets and outlets this way means, however, that only part of the plate surface is used efficiently for heat transfer, since great flow rate differences arise between a partial flow taking the shortest way from the inlet to the outlet and a partial flow describing on its way from the inlet to the outlet a curved path via the opposite edge of the plate.
There are other applications where, as a standard solution, only one type of metal plate is used, but different types of gaskets in every second interspace, to constitute the whole plate heat exchanger. In this type of structure, every second plate is rotated through 180 degrees in the plane about a centre line extending, accordingly, perpendicularly to the plane of the plate. This means, just as in the above case, that the different ports have to be of the same size. In this plate design, different kinds of stiffening means in the form of special gaskets or weigh belts are also often used. However, this entails additional costs for manufacture and mounting of the stiffening means. In addition, these stiffening means often have a detrimental effect on the functioning of the heat exchanger since they interfere with the flow in an undesired way.
A further example of prior art is to be found in GB-A-2,121,525, which discloses a plate heat exchanger in the form of an evaporator provided with two different types of plates in a plate pack.
There are, however, a large number of applications where the above special cases are not applicable. For example, they cannot be used when one or both fluids undergo a phase transformation. There are evaporation and/or condensation processes, for example, where a liquid is transformed into vapour and vapour is transformed into liquid, which requires different sizes of the inlets and outlets for the respective fluids (see the above-mentioned U.S. Pat. No. 5,226,474).
Thus, there is no general technique for reducing to only one the number of plate types in one and the same plate heat exchanger. The attempts at solving the problem proposed have either been limited to very special applications or require special stiffening means and gaskets resulting in a more expensive and poorer construction, which means that the economic benefit of only one type of plate is lost.
There is above all a great need for a reduced number of plate types in various forms of condensation and evaporation applications, since these require relatively large plates in order to achieve an efficient heat exchange even if one of the fluids is in the vapour phase. The need is even more pronounced in connection with large-scale industrial operation processes.
The object of the invention is to provide a solution to the above problems.
More specifically, the primary object of the invention is to provide a heat exchanging plate, which is constructed in such a way that a plate heat exchanger can be manufactured at the lowest possible cost, the flow of each of the two heat exchanging fluids in the plate heat exchanger being as evenly distributed as possible in the respective plate interspaces. Furthermore, the construction shall be such that the heat transfer plate, when it is assembled with similar plates in a plate heat exchanger, is able to resist large pressure differences between the heat exchanging fluids. In addition, the construction should be such that the above advantages may be obtained even if one or both fluids are intended to undergo a phase transformation during the heat exchange in the plate heat exchanger, and each heat exchanging plate, therefore, has to be provided with an inlet port area of a different size than the outlet port area for each of the fluids, or only one of the fluids.
According to the invention, this object is achieved by means of a heat transfer plate of the type described by way of introduction and having the features defined in claim 1.
The invention also concerns a plate pack for use in a plate heat exchanger, which comprises a number of heat transfer plates as above and also has the features defined in claim 17.
Furthermore, the invention concerns a plate heat exchanger, which has the features defined in claim 21.
Finally, the invention also concerns use of a heat transfer plate of the type described above.
Preferred embodiments of the invention will be apparent from the dependent claims.
The fact that the plate has a symmetry line, which extends in the main flow direction of the heat exchanging fluids from said first edge to said second edge of the plate and relative to which the plate""s heat transfer portion, sealing portions and ports to be passed by each of said fluids are symmetrically arranged results in a plate that can be flipped about its symmetry line and brought to abut against another identical plate for forming a pair of plates or a plate pack with several plates of one and the same type.
To ensure that the plate, when it is assembled with identical plates in a plate heat exchanger, is able to resist large pressure differences between the heat exchanging fluids, said elevations and depressions are so arranged relatively to said symmetry line that when two identical plates are brought to abut against each otherxe2x80x94one of the plates being rotated through 180 degrees about the symmetry line relative to the otherxe2x80x94said elevations of the plates will form distance means between the plates in a number of places distributed over the heat transfer portions of the plates.
Owing to the symmetry properties only one type of plate is required, which means that only one set of pressing tools is needed, which in turn implies smaller investments compared to prior art.
The above symmetry requirements are such that the inlet ports and outlet ports for the respective fluids can be given a different shape and total area, which means that the plate can also be used in plate heat exchangers where one or both fluids undergo a total or partial phase transformation.
The symmetric location of the ports also has the advantage that the fluid flows will cover the major part of the plate surfaces instead of only flowing along one or the other of the edges of the plates. Furthermore, the symmetric location results in a smaller difference in flow paths and thereby in flow rates between different partial flows of a fluid in one and the same plate interspace. This ensures a high efficiency of the heat exchanger, meaning that for a given heat-exchanging task smaller and, thus, cheaper plates can be used. Moreover, the risk of the fluid flow in any portion of the plates becoming so small that this portion xe2x80x9cruns dryxe2x80x9d, which could mean that one of the fluids gets burnt and sticks to the plate at this portion, is reduced.
The plates are advantageously formed with circumferential ridges delimiting the desired flow areas on the respective sides of the plates. These ridges form both distance means and sealing means.
A preferred way of ensuring that the distance means are symmetric point by point with respect to the symmetry line is to provide the plate with elongated corrugations that form ridges arranged asymmetrically relative to the symmetry line.
When plates of the type mentioned above are used in a plate pack, in which every second plate is flipped through 180 degrees about said symmetry line relative to the other plates, a plate pack is obtained which is well suited for welding in pairs, but which may also be used in combination with an optional sealing system. Flipping every second plate about the symmetry line results in the ports and distance means coinciding with the corresponding elements on an adjacent plate. The plates will be arranged so that the first side of each plate faces the first side of an adjacent plate and the second side of each plate faces the second side of an adjacent plate.